Even as a boy Jim Clark's ambition was always to work in film production. At the age of ten his father bought him a Pathé Ace 9.5mm film projector, "the great toy of my life, which changed it... It one day occurred to me that in some way the scenes changed from one angle to another, and I became interested in the reason why. Then I became interested in the manipulation of images to tell a story." He ordered films from the Wallace Heaton film library, read everything he could about cinema, and established a school film society. Clark then worked in his father's printing business for three years but continued to pursue his passion by remaining involved with the Federation of Film Societies and regularly attending festivals and National Film Theatre screenings. At these events Clark got to know both established film-makers and aspiring ones such as Lindsay Anderson and Karel Reisz.

A brief stint as a runner in a documentary production company was followed by an unexpected invitation to join Ealing Studios as a cutting room assistant. Clark assisted sound editors Gordon Stone and Mary Habberfield and editor Jack Harris. His first break proved to be a false start. Basil Wright asked Clark to edit a children's film he was producing. Unable to find work in mainstream features after this detour, Clark returned to assisting Jack Harris, who eventually recommended him to edit Surprise Package (d. Stanley Donen, 1960). In the 1960s Clark edited two films for Jack Clayton and then commenced a longstanding creative partnership with John Schlesinger on Darling (1965). Schlesinger describes Clark as someone "who has saved my bacon many times". Between the mid-1960s and mid-1970s Clark directed four eclectic low-budget features, but returned to full-time editing because he found this role more satisfying. He did however enjoy directing some documentaries for Granada Television during this period.

Clark believes an editor's role should be a creatively supportive one. He also feels an editor should, when asked to, advise on performances during production. Clark has come to be seen, for example by producer David Puttnam in the 1980s, as someone able to solve problems and give tactful assistance to new features directors: 'people do come to me for advice on scripts, and looking at cuts, and seeing films and advising and commenting.' Since the late 1960s Clark has often worked in Hollywood, although in recent years he has collaborated with British directors Michael Caton-Jones, David Leland, Peter Cattaneo and, most notably, Mike Leigh (Vera Drake, 2005; Happy-Go-Lucky, 2008). In 2005 Clark received a Lifetime Career Achievement Award from the American Cinema Editors' organisation (ACE).

Clark considers one of his Hollywood projects, The Day of the Locust (US, d. John Schlesinger, 1975), an excellently directed, "incredibly underrated film". Clark is particularly proud of his work on its apocalyptic finale, set at a Hollywood premiere. Cuts on violent action accentuate the force of a panicked crowd rioting, fires spreading, and wreckage falling. A series of dissolves which begin and end with shots of blinding bright light intensify the delirium experienced by wounded art director Tod (William Atherton) as chaos engulfs him.

Roy Perkins/Martin Stollery, British Film Editors: The Heart of the Movie (BFI Publishing, 2004)